1. Field of the Invention
This invention relates to the field of nickel base superalloys used in applications where mechanical stresses are encountered at high temperatures. This invention is also related to the field of directionally solidified eutectics wherein an alloy of approximately eutectic composition may be directionally solidified so as to produce an aligned microstructure having anisotropic mechanical properties. The alloy of the present invention is a nickel base superalloy of approximately gamma prime-alpha eutectic composition which may be directionally solidified to produce articles having exceptional mechanical properties combined with adequate oxidation and sulfidation resistance at elevated temperatures.
2. Description of the Prior Art
It is now known that certain eutectic alloys respond to proper directional solidification conditions to produce useful second phase aligned microstructures as described in the patent to Kraft, U.S. Pat. No. 3,124,452. In a patent to Thompson, U.S. Pat. No. 3,554,817 there is described a promising pseudo binary eutectic alloy occurring between the inter-metallic compounds Ni.sub.3 Al and Ni.sub.3 Cb which responds to plane front solidification to produce a casting characterized by an aligned lamellar microstructure. As so cast this combination provides one of the strongest nickel base alloys known although its ductility is less than that of most nickel base superalloys. As is the case with most nickel base alloys, however, improvements in oxidation and sulfidation resistance are desired to permit maximum utilization of the strength characteristics of the alloy at very high temperatures i.e. &gt;1000.degree. C. And even though in its usual application a given alloy may be provided with surface protection for increased oxidation erosion resistance, improvements in corrosion resistance in the underlying substrate are nevertheless desirable.
In a prior patent to Thompson and Lemkey, U.S. Pat. No. 3,564,940 there is described a class of compositions which solidify according to the monovariant eutectic reaction providing aligned polyphase structures including such systems as the ternary alloys identified as cobalt-chromium-carbon and nickel-aluminum-chromium. The advantage of compositions of this nature is that the desired microstructure can be achieved over a range of compositions in a given system. This provides a substantial increase in the freedom of selection of compositions permitting increased optimization of properties. In U.S. Pat. No. 3,671,223 the concept has been further developed to include those systems which solidify according to the multivariant eutectic reaction where two or more solid phases (N) crystalize simultaneously from the liquid consisting of (N+2) or more components.
U.S. Pat. No. 3,617,397 to Maxwell, assigned to the present assignee discloses a nickel base superalloy which contains 8 percent aluminum and 18 percent molybdenum. This alloy is far from the eutectic point and the patent does not disclose the benefits available in the directionally solidified eutectic.
U.S. Pat. No. 3,793,010 to Lemkey and Thompson discloses a eutectic article which consists of a gamma-gamma prime matrix with an aligned delta second phase.
The most accurate previous work on the nickel-aluminum-molybdenum alloys is described in "The Form of the Equilibrium Diagrams of Ni-NiAl-Mo Alloys" Academy of Sciences USSR, 132, May-June 1960, pp 491-495, however this reference does not discuss the eutectic reaction in question.